Most animals, including humans, carry an internal lunar clock, tuned to the 29.5-day rhythm of the Moon. It guides sleep, reproduction and migration of many species. But in the age of artificial light, that ancient signal is fading - washed out by the glow of cities, screens and satellites.
Author
- Stefano Arlaud
PhD candidate in Time Processing and Metacognition of Time Processing, SBBS, Queen Mary University of London
Just as the circadian rhythm keeps time with the 24-hour rotation of the Earth, many organisms also track the slower rhythm of the Moon. Both systems rely on light cues, and a recent study analysing women's menstrual cycles shows that as the planet brightens from artificial light, the natural contrasts that once structured biological time are being blurred.
Plenty of research suggests the lunar cycle still influences human sleep. A 2021 study found that in Toba (also known as Qom ) Indigenous communities in Argentina, people went to bed 30-80 minutes later and slept 20-90 minutes less in the three-to-five nights before the full Moon.
Similar, though weaker, patterns appeared among more than 400 Seattle students in the same study, even amid the city's heavy light pollution. This suggests that electric light may dampen but not erase this lunar effect.
The researchers found that sleep patterns varied not only with the full-Moon phase but also with the new- and half-Moon phases. This 15-day rhythm may reflect the influence of the Moon's changing gravitational pull, which peaks twice per lunar month , during both the full and new Moons, when the Sun, Earth and Moon align. Such gravitational cycles could subtly affect biological rhythms alongside light-related cues.
Laboratory studies have supported these findings. In a 2013 experiment , during the full Moon phase participants took about five minutes longer to fall asleep, slept 20 minutes less, and secreted less melatonin (a hormone that helps regulate the sleep-wake cycle). They also showed a 30% reduction in EEG slow-wave brain activity - an indicator of deep sleep.
Their sleep was monitored over several weeks covering a lunar cycle. The participants also reported poorer sleep quality around the full Moon, despite being unaware that their data was being analysed against lunar phases.
Perhaps the most striking evidence of a lunar rhythm in humans comes from the recent study analysing long-term menstrual records of 176 women across Europe and the US.
Before around 2010 - when LED lighting and smartphone use became widespread - many women's menstrual cycles tended to begin around the full Moon or new Moon phases. Afterwards, that synchrony largely vanished, persisting only in January, when the Moon-Sun-Earth gravitational effects are strongest .
The researchers propose that humans may still have an internal Moon clock, but that its coupling to lunar phases has been weakened by artificial lighting.
A metronome for other species
The Moon acts as a metronome for other species. For example, coral reefs coordinate mass spawning events with precision, releasing eggs and sperm under specific phases of Moonlight.
In a 2016 laboratory study , researchers working with reef-building corals (for example A. millepora) replaced the natural night light cycle with regimes of constant light or constant darkness. They found that the normal cycling of clock-genes (such as the cryptochromes) was flattened or lost, and the release of sperm and eggs fell out of sync. These findings suggest lunar light cues are integral to the genetic and physiological rhythms that underlie synchronised reproduction.
Other species, such as the marine midge Clunio marinus, use an internal "coincidence detector" that integrates circadian and lunar signals to time their reproduction precisely with low tides. Genetic studies have shown this lunar timing is linked to several clock-related genes - suggesting that the influence of lunar cycles extends down to the molecular level.
However, a 2019 study found that the synchrony of wild coral spawning is breaking down. Scientists think this may be due to pollutants and rising sea temperatures as well as light pollution . But we know that light pollution is causing disruption for many wildlife species that use the Moon to navigate or time their movements.
Near-permanent brightness
For most of human history , moonlight was the brightest light of night. Today, it competes with an artificial glow visible from space. According to the World Atlas of Artificial Night Sky Brightness, more than 80% of the global population - and nearly everyone in Europe and the US - live under a light-polluted sky (one that is bright enough to hide the Milky Way).
In some countries such as Singapore or Kuwait, there is literally nowhere without significant light pollution. Constant sky-glow from dense urban lighting keeps the sky so bright that night never becomes truly dark.
This near-permanent brightness is a by-product of these countries' high population density, extensive outdoor illumination, and the reflection of light off buildings and the atmosphere. Even in remote national parks far from cities, the glow of distant lights can still be detected hundreds of kilometres away .
In cognitive neuroscience, time perception is often described by pacemaker-accumulator models , in which an internal "pacemaker" emits regular pulses that the brain counts to estimate duration. The stability of this system depends on rhythmic environmental cues - daylight, temperature, social routines - that help tune the rate of those pulses.
Losing the slow, monthly cue of moonlight may mean that our internal clocks now run in a flatter temporal landscape, with fewer natural fluctuations to anchor them. Previous psychological research has found disconnection from nature can warp our sense of time.
The lunar clock still ticks within us - faint but measurable. It shapes tides, sleep and the rhythms of countless species. Yet as the night sky brightens, we risk losing not only the stars, but the quiet cadence that once linked life on Earth to the turning of the Moon.
Stefano Arlaud does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
